1. Field of the Invention
The present invention is in the field of ventricular assist devices, and of artificial prosthetic conduits used for transporting blood in the circulatory system of a living organism. More particularly, the present invention relates to a ventricular assist device which includes a continuous unitary blood-contacting membrane defining a variable-volume cavity, expansion and contraction of which is effective to pump blood; and to a valved blood conduit for communicating blood to or from the variable-volume chamber, and having liquid-impermeable membrane or inner wall defining a blood-contacting surface within the conduit. The inner wall of the conduit sealingly engages the unitary blood-contacting membrane of the variable-volume chamber without a blood-contacting gasket or sealing member, so that only a single material-surface transition is experienced by the flowing blood upon entry into or outflow from the ventricular assist device.
Also, the present invention relates to an artificial conduit having therein a prosthetic bio-material valve structure, and associated conduit structure for ensuring a substantially laminar central jet of blood flow through the conduit and valve structure, while also ensuring that flow disruption is minimized, and that no blood stagnation or stasis volumes are formed downstream of or behind the valve structure. Still more particularly, the present invention relates to such a valved blood conduit having woven and/or knitted filamentary fabric walls which are impregnated outwardly with a biologically-compatible impermeable material so that the conduit walls are impermeable to blood, while the inner surface of the conduit wall remains textured or porous to promote the growth of a stable biological interface. Provision is made for sealingly connecting the valved blood conduit to other blood-carrying components without disruption of smooth and stasis-free blood flow. The connecting provisions also minimize the number of blood-contacting material-surface transitions, and provide for accommodation without loss of sealing integrity of dimensional changes which will occur at the connections after implantation of the valved conduit and assist device. These dimensional changes will occur as a transitional collagen or other biodegradable coating of the conduit is absorbed, as components of the valved conduit and adjacent structure take a set with the passage of time after surgical implantation, and as a biological interface is formed on the blood-contacting surfaces by the host's circulatory system.
2. Related Technology
Ventricular assist devices have become increasingly recognized as potentially able to allow patient's whose natural heart is diseased or has been injured by trauma or heart attack, to recover and continue life, either while their natural heart heals, while awaiting a heart transplant, or even on a long-term basis with the extended aid of the ventricular assist device.
Particularly, left-ventricular assist devices (LVAD) are recognized as potentially very valuable for assisting patients who suffer from congestive heart failure. More than two and one-half million Americans suffer from congestive heart failure. Recently, a National Institutes of Health study estimated that as many as thirty-five thousand people could be candidates for use of a left-ventricular assist device. At present, the conventional ventricular assist devices are used for patients who are waiting for a heart transplant (a so-called, "bridge to transplant"), for patients whose natural heart is of such poor condition that the patient cannot be removed from a heart-lung machine without providing some assistance to the patient's heart following otherwise successful open-heart surgery, and for patients suffering from massive heart attacks that lead to circulatory collapse. The conventional left-ventricular assist devices are not generally considered to be viable candidates for long-term utilization outside of the clinical environment for a plurality of reasons.
Most heart disease involves the left ventricle of the heart. This pumping chamber is generally known as the workhorse of the heart. A patient with a non-functioning right ventricle can survive quite successfully provided that their pulmonary blood flow resistance is low enough to allow circulation through the lunge; and the rest of the body entirely as a result of the efforts of the left ventricle. However, collapse of the left ventricle is most often fatal. An LVAD is able to fully take over the function of this ventricle, thus perfusing the body with oxygen-rich blood. The LVAD attaches to the patient's natural heart, and to a natural artery, and can be removed if the natural heart recovers.
Blood flow in the LVAD is effected by expansion and contraction of a variable-volume chamber. One-way valves associated with the inflow and outflow ports of the LVAD provide for blood flow into the variable-volume chamber during expansion, and for blood flow out of this chamber, usually to the ascending thoracic aorta. These one-way flow valves may be constructed as part of the LVAD itself, or may be disposed in the blood-flow conduits which connect the LVAD to the heart and aorta. A pair of conduits respectively connect the inlet port of the assist V device to the left ventricle and the outlet port to the major artery which is to receive the blood flow from the device.
As described above, artificial blood conduits have become a valuable tool of modern medicine. One use of such artificial blood conduits is as a temporary or permanent prosthetic artery. Another use is in the connection of temporary blood pumps, or ventricular assist devices, between the left ventricle of the heart and a major artery. In such a use, the demands on the artificial blood conduit are great. The artificial conduit must deal with the pulsatile blood flow created by the host's own heart, as well as with the flow, pressure, and pulsations created by the assist device. The artificial conduit must function within or outside of the host patient's body, and not introduces or allow the entry of bacterial or other contamination into the host's body or circulatory system. Also, the artificial conduit must be connected to both the heart, or to a major artery of the host's circulatory system in order to allow connection of both the artificial conduit, and also of the ventricular assist device or pump.
A persistent problem with artificial blood conduits has been the provision of a valving device of the one-way type in these conduits so that a ventricular assist device can achieve pulsatile blood flow in response to the expansion and contractions of a variable-volume chamber of the assist device.
A conventional artificial blood conduit is know in accord with U.S. Pat. No. 4,086,665, issued May 2, 1978, to Poirier. The blood conduit of the Poirier patent is believed to include an internal convoluted fabric tube of essentially circular cylindrical configuration throughout its length. This inner fabric tube is carried within an outer tube, which is also convoluted over part of its length. The inner tube is porous while the outer tube is liquid impervious. A tri-foliate valving structure is provided in the conduit to ensure unidirectional blood flow in the conduit. This tri-foliate valving structure is taught by the Poirier patent to be a porcine xenograft, sutured into the fabric of the inner tube. A circular support ring may be disposed outside of the inner tube wall to assist in support of the xenograft. Provision is made for connection of the artificial blood conduit of Poirier to other blood-carrying structure, and to the vascular tissue or heart tissue of the host via suture rings. Essentially, Poirier teaches that the valved conduit may be connected to other blood-carrying structure by means of flanged connections using gasket-sealed interfaces and threaded collars which engage onto threaded portions of the adjacent conduit or other blood-carrying structure.
With the artificial blood conduit taught by the Poirier patent, the conduit structure itself is quite bulky, being composed of several concentric structures or elements, some of which are spaced apart radially from one another. As a result, the Poirier conduit has a considerable wall thickness built up by all of these individual wall elements. Additionally, the inner lumen or passageway of this artificial conduit does not provide for elimination of blood flow stagnation or stasis downstream of the tri-foliate valve structure. Accordingly, the stagnant blood may clot or may adhere to the walls of the conduit, to be shed eventually as emboli in the circulatory system of the host. Also, the annular space between the inner porous conduit and the outer impervious conduit may harbor bacterial contamination, and provide a site for bacterial growth and infection which is hidden from the patient's immune system.
A conventional bio-material xenograft valve is known in accord with U.S. Pat. No. 4,247,292, issued Jan. 27, 1981, to W. W. Angell. The Angell patent is believed to disclose an externally-stented natural tissue valve for heart implantation in which the natural xenograft tissue is sutured to a fabric covered plastic stent. The valve is secured into a patient's heart by sutures between the suture ring and the heart tissue. There is no artificial conduit which is valved by the device of Angell.
Another conventional artificial conduit is disclosed by U.S. Pat. No. 5,139,515, issued Aug. 18, 1992 to F. Robicsek. The Robicsek patent is believed to disclose an artificial aortic root portion which includes a convoluted wall formed with sinuses generally aligned with the leaflets of the natural tri-foliate valve of the patient's heart. As so configured, it is asserted that the blood flow "recoil" downstream of the valve leaflets will assist in their closing, resulting in a more natural valve function, with reduced regurgitaition. However, the artificial aortic root portion taught by Robicsek includes out-pouchings, or sinuses, which are themselves formed with corrugations or convolutions like the rest of the artificial conduit. These convolutions at the sinuses themselves may contribute to the formation of small localized turbulent zones, or to the formation of stasis or stagnation volumes where blood flow is slowed or stopped. In either case, the fluid flow dynamics of the artificial conduit suggested by the Robicsek patent is highly questionable because it may cause the formation of clots which are eventually shed as emboli in the circulatory system.
Yet another artificial valve is known in accord with British patent specification No. 1315845, of B. J. Bellhouse, the complete specification for which was published on May 2, 1973. The Bellhouse specification is believed to disclose an artificial valve for implantation within the natural aortic root, with a ring part formed of silicone-coated uncut polyethylene terephthalate fabric. The cusps of this valve are formed of woven and/or knitted material of the same type of polyethylene terephthalate fabric, which is also coated with silicone rubber. However, the valve of Bellhouse is implanted into the natural aortic root, with the natural sinuses present, and does not include a prosthetic conduit for blood flow.
A persistent problem with all of the above-identified conventional devices, and with others which are known also in the art, is the rather high number of material-surface transitions, or changes in the material across which the patient's blood must flow in passing through the devices. For example, in the artificial blood conduit of Poirier, disclosed in the '665 patent, the flowing blood is exposed to at least nine different surfaces in flowing through this device. These different surfaces include the tissue surfaces of the porcine xenograft, the sutures which secure this graft, the fabric inner conduit, the gasket surfaces at the ends of the valved conduit, and the end connectors to which the fabric inner conduit connects. When the entire ventricular assist device of Poirier is considered, several additional blood-contacting surfaces of different materials, or material-surface transitions, must also be added to this list. Each of these blood-contacting material-surface transitions represents a potential source of turbulence in the flowing blood if the adjacent surfaces do not align perfectly with one another.
Additionally, the flowing blood may not have the same affinity for creating a stable biological interface with each of the various materials. That is, the material surfaces may have a differing degrees of surface porosity, of surface roughness, of surface energy, or of bio-compatibility with the host, for example. Consequently, with the passage of time, the biological interface between the flowing blood and the artificial, "not self" surfaces will be laid down with discontinuities, or with changes in tenacity of attachment to the underlying artificial surfaces, for example, at these material-surface transitions in the device. Each of these discontinuities or changes in tenacity of attachment of the biological interface with the underlying artificial structure represents an opportunity for a portion of the interface to slough off to become an emboli in the circulating blood. Also, blood may clot at these unstable interfaces, also representing a risk of forming emboli in the blood.